the first time I changed the air filter on my 1500 I noticed oil on the left side of the air box (inside). I cleaned it all up, put a new filter on, then 4000 miles later I opened up the air box to inspect the filter and its all back, even some oil on the filter. I know the crank case vent system is on the lower left side and its probably misting oil on that side and on the filter. I'm not over filling the engine with oil, the clear tubes are clean and everything seems to run good, but I know having oil on 1/3 of the air filter and the left carb probably sucking some oil back in and maybe running rich on that side isn't doing gas mileage any good and we know it's just not right. Any thoughts anyone ??

Chickenlegs wrote:the first time I changed the air filter on my 1500 I noticed oil on the left side of the air box (inside). I cleaned it all up, put a new filter on, then 4000 miles later I opened up the air box to inspect the filter and its all back, even some oil on the filter. I know the crank case vent system is on the lower left side and its probably misting oil on that side and on the filter. I'm not over filling the engine with oil, the clear tubes are clean and everything seems to run good, but I know having oil on 1/3 of the air filter and the left carb probably sucking some oil back in and maybe running rich on that side isn't doing gas mileage any good and we know it's just not right. Any thoughts anyone ??

If you're positive the engine is not overfilled, there could be a number of different causes:

The first two would likely result more in oil being pumped up there, not misted, so I would check the third first. Check the compression in all cylinders, both dry and wet (after injecting small amount of oil into each cylinder).

when you say restricted oil drain path, are you talking about the black curved rubber hose next to the clear one? just out of curiosity how much over full would it take to puke it back up 1/4 qt. 1/8 qt ?.

Chickenlegs wrote:when you say restricted oil drain path, are you talking about the black curved rubber hose next to the clear one? just out of curiosity how much over full would it take to puke it back up 1/4 qt. 1/8 qt ?.

No, I'm talking about a restricted oil drain path inside the engine, where oil drains back to the sump and is then picked up by the scavenge pump. Basically if the main oil pump is pumping oil in and it isn't being drained properly, the oil level in the engine rises up.

And 1/4 qt definitely wouldn't be enough to cause that. I would definitely look at cylinder compression before anything else. The hose leading from the crankcase to the air filter box is designed to allow cylinder blow-by to escape the cylinder block (instead of blowing out seals). It is environmentally insensitive to just vent these gases, so instead they are routed up to the air box, where they are sent back through the intake and burned once again, this time in hopes they make it out the exhaust.

When you have poor compression, AND this poor compression is caused by worn piston rings or cylinder walls, it means the high pressure gases are getting past the rings (blow-by) and into the crankcase. With enough poor compression, there could be enough pressure getting in there to blow an oil mist up into the air box.

Put a compression tester on each cylinder and test the compression (crank the engine with the throttle wide open). Brand new from the factory you should see 213 psi on each cylinder. As an engine wears this number will drop. For the most part all the pistons will (and should) wear evenly, so this number should be more or less the same on every cylinder. If I saw 170 psi on 5 cylinders and 140 psi on one cylinder, I would be worried.

We have to determine the cause of loss of compression. There are two possibilities: leaky rings and leaky valves. To determine which one it is, we introduce some oil into the cylinder to seal the rings. On our flat boxer engines this is tough to do, because gravity is working against us. The best method is a fogging - if you have a light weight oil that will spray mist from a spray bottle, this is great. However you don't want to introduce TOO much fluid, as this will reduce the volume of the cylinder and artificially increase the compression reading.

Once you've wetted the cylinder, test the compression. If the number stays more or less the same, then your problem is going to be leaky valves. If the numbers go up, then you're looking at a ring problem - and likely the source of your oily air box.

ok you're kinda freaking me out. the bike does run very good, starts right up, no smoke, now has 33,000 miles, gas mileage is between 34 and 40 depending on how fast I ride. I did do a lot of high altitude riding (14,000 ft) my wife and I spent a week in the Colorado mountains without any problems and the bike ran strong even up there, that's where we do most of our riding anyway, lucky us. I will do a compression test and a leak down test, just to know the truth. What's strange is you would think that the clear tube would partially fill up with oil, I cleaned it once (about a half inch of stuff) when I first got the bike and has stayed clean ever since (5000 miles worth). thank you for all your thoughts and suggestions. I am open to more conversation.